Deployment: VDT0410

Surface-Ocean Lower-Atmosphere Studies Air-Sea Gas Experiment
Phytoplankton blooms, either natural or stimulated, provide effective natural laboratories in which to study the pronounced biogeochemical fluxes and gradients associated with their evolution and decline. These phytoplankton-mediated signals are mainly expressed in the ocean, but also result in enhanced fluxes of carbon dioxide (CO2), dimethylsulfide (DMS) and other biogenic gases across the air-sea
interface.

The Southern Ocean is a net sink region for atmospheric CO2, yet uncertainties remain in the strength of this sink because few measurements of the efficiency of ocean-atmosphere gas exchange have been made under turbulent windy open-ocean conditions.

During SAGE, in a similar fashion to SOIREE in 1999, we proposed to stimulate a phytoplankton bloom through addition of iron fertiliser to iron-limited Sub-Antarctic waters. The fertilisation was marked with the addition of two inert dissolved gas tracers, sulfur hexafluoride (SF6) and Helium-3 (3He), creating a lagrangian patch/dual-tracer study with the tracer SF6 providing a control volume, vertical and lateral diffusion rates and estimates of air-sea gas exchange in association with 3He. The enhanced gas fluxes associated with the bloom should provide optimal
conditions for measuring the rate of gas exchange and the key physical processes governing the exchange. These processes include near-surface turbulence (typically generated by breaking waves), temperature microstructure, stratification, wave field, wave breaking and wind speed. In conjunction with these patch scale and surface
physics measurements, the micrometeorologic al relaxed eddy accumulation technique (REA) was deployed to make direct atmospheric measurements of gas fluxes. A combination of gas concentration measurement and REA flux potentially allows the efficiency of gas exchange to be calculated at the local scale. These local scale  measurements can be compared with exchange rates derived from the dual tracer technique for the larger labelled patch.

Experimental goals
Determine drivers and controls of ocean-atmosphere gas exchange quantifying:
- biological production and utilisation of climatic relevant gases in particular CO2 and DMS)
- in the surface ocean
- physical control of exchange across the interfaces of the surface mixed layer
- production of aerosols resulting from interaction of biological and physical processes

Objectives:
This experiment combined seven main research objectives considering:
1. quantification of gas transfer fluxes and velocities for a variety of gases
2. physical processes affecting gas transfer
3. ecosystem interactions controlling dissolved DMS concentration and CO2 removal
4. the impact of iron availability upon phytoplankton productivity and its influence upon dissolved
- gas concentration
5. the impact of photochemistry in the surface ocean on dissolved gas concentration and air-sea exchange
6. the fate of DMS in the atmosphere and aerosol condensation nuclei production from chemical
- transformation in the atmospheric boundary-layer.
7. Role of aggregation in the timing and magnitude of export processes

Additional objectives were the:
1. servicing of NIWA biophysical moorings: 41°11.28'S 178°28.62'E Northern Biophysical Mooring
- (NBM) and approximately 46°38.202'S 178°33.486'E Southern Biophysical Mooring (SBM)
2. final release of 2 Carioca Buoys at SBM

SAGE Cruise Track from SST data